This invention relates to an engine control system and method and more particularly to an improved feedback control system and method for a marine propulsion engine.
The control of an engine is extremely important in ensuring good running. In addition to providing the appropriate and desired output for the engine under a wide variety of running conditions, engine control is important in ensuring good fuel economy and also effective exhaust emission control. Therefore, it has been proposed to provide engines with sensors which sense the actual air-fuel ratio of the engine and make adjustments through a feedback control system so as to ensure that the air-fuel ratio is obtained at the desired amount.
Although this theory may be relatively simple, the application of this principle is not quite as straightforward. The problem is particularly acute in conjunction with marine propulsion engines. Unlike land vehicles, very nature of a watercraft and its systems can have a greater effect on the engine performance than a land vehicle. That is, many characteristics of the vehicle condition can have a significant effect on the engine itself and the proper control for the engine. This is also true if feed back control is employed.
For example the exhaust gases from a marine propulsion engine are frequently discharged, under at least some running conditions, below the level of water in which the associated watercraft is operating. This underwater exhaust gas discharge is employed so as to assist in exhaust silencing.
With land vehicles the condition at the exhaust discharge generally is maintained fairly constant under all running conditions. That is, there are not outside factors which will affect the ability of the exhaust system to discharge the exhaust gases under most normal conditions. However, when the exhaust gases are discharged beneath the water, as with a marine propulsion engine, then the condition at the exhaust outlet can vary significantly.
One way in which the condition can vary is that the depth of the underwater exhaust gas discharge is not constant during the running of the watercraft that is powered by the engine. In fact, the variations are not even linear in relation to speed. The reason for this is that many watercraft use so-called "planing-type" hulls that operate fairly deeply submerged under low-speed conditions. However, as the speed increases and the watercraft goes on plane, the water level can change suddenly relative to the location of the exhaust gas outlet. This can have a significant effect on the performance.
Also it is the practice to change the trim of the propulsion unit during running. This is done to achieve the optimum angle of attack for the propeller and the optimum angle also does not vary linearly with speed. The trim angle also affects the exhaust path to reach the atmosphere. Thus trim angle also affects engine performance.
It is, therefore, a principal object of this invention to provide an improved feedback control for a marine propulsion engine that takes into account the watercraft conditions such as depth of running and trim angle.
It is a further object of this invention to provide an improved feedback control for a marine propulsion engine wherein the depth of submersion of the exhaust outlet and its trim angle is factored into the feedback control variables.
In addition to the depth of the exhaust outlet in marine applications, the exhaust gases are frequently discharged, particularly under high-speed operation, through a through-the-propeller or through-the-hub exhaust gas discharge. However and as has been noted, when the watercraft is propelled by a propeller, it is the normal practice to mount the propeller so that its trim position can be adjusted. That is, the angle of axis of rotation of the propeller relative to the transom is varied. This is done both in outboard motors and in the outboard drive portion of an inboard/outboard drive. These types of drives are referred to generally as marine outboard drives.
In addition to changing the depth of submersion, the trim adjustment changes the angle at which the exhaust gases are discharged relative to the water level. Hence, the back pressure on the exhaust gases can vary with the trim angle, even if the depth is maintained uniform.
It is, therefore, a still further object of this invention to provide an improved feedback control for a marine propulsion engine wherein the trim angle of the drive is considered in setting the feedback control.
With water vehicles and those having under water exhaust discharges the back pressure on the exhaust outlet also varies even if the depth and trim are not adjusted. The engine frequently gains speed faster than the boat. Hence on acceleration the amount of exhaust gasses discharged increases faster than the watercraft speed. As the watercraft speed increases the pressure at the exhaust outlet will go down, other factors being held equal. However during acceleration and at least at the initial phases of acceleration the exhaust back pressure may go up even more than expected.
It is therefore still another object of this invention to provide a feed back engine control system and method that is responsive to transient watercraft conditions.
The types of feedback control employed generally control the air-fuel mixture by controlling the amount of fuel in response to the output of the sensor. The sensor is frequently an exhaust sensor, such as an O.sub.2 sensor, that emits a signal which is indicative of the richness or leanness of the mixture. If the feedback control is done only as a function of the output of this sensor, the control may not be as effective because it disregards the other factors noted above.
It is, therefore, a still further object of this invention to provide an improved feedback control system and method for a marine propulsion engine that takes into account watercraft conditions that will affect engine performance.
In addition to trim adjustment, many watercraft also permit or effect height adjustment of the propulsion unit. From the foregoing discussion it should be apparent that height changes will also affect engine performance.
It is, therefore, a still further object of the invention to provide an engine feed back control system and method that compensates for changes in the height of the propulsion unit.
It has also been discovered that merely changing the amount of fuel supplied to the engine does not necessarily achieve the complete results desired. For example, with spark-ignited engines it has been found that the spark advance should also be altered in response to the feedback signal. Like the fuel-ratio variation, spark advance control also depends upon factors of watercraft condition that may affect engine operation.
It is, therefore, a still further object of this invention to provide an improved feedback control system for the spark timing which is dependent upon factors affecting exhaust performance such as conditions of the powered watercraft.
In controlling the air-fuel ratio, it is necessary, of course, to provide a good indication of air flow to the engine so that the fuel flow that is varied can be varied in proportion to air flow. With two-cycle engines, a type of engine frequently employed in marine propulsion applications, it has been noted that the amount of air flow to the engine can be accurately determined by measuring the pressure in the crankcase chamber at particular crank angles. Therefore, many engine control systems employ crankcase pressure sensors so as to control the amount of fuel supplied.
It has been discovered, however, that the exhaust back pressure also can vary the accuracy of the crankcase pressure sensor in determining the accurate airflow to the engine.
It is, therefore, a still further object of this invention to provide an improved feedback control system where the airflow is measured by crankcase pressure and wherein adjustments are made in the air flow determination based upon factors which affect the back pressure in the exhaust system.
In engines that have plural cylinders, frequently the engine is supplied with an exhaust system that includes a manifold that collects exhaust gases from a number of exhaust ports and delivers it to the atmosphere through a common exhaust gas opening of the exhaust system. With these systems and with particular applications the distance between the exhaust gas opening and the exhaust ports of the individual cylinders may be different. This is a problem that is particularly acute in conjunction with marine propulsion applications due to the compact nature of the exhaust system that must be employed for these applications.
This difference in length can be particularly significant in conjunction with two-cycle engines, wherein exhaust pulses can have a significant effect on the charging of the individual cylinders. This is caused in part by the substantial overlap between the opening of the scavenge port and the closing of the exhaust port. When utilizing a feedback control, the collected flow of the exhaust gases is normally measured, and this is used for determining the air-fuel ratio. As a result, although the average for the system may be acceptable, individual cylinders are not supplied with the appropriate air-fuel mixture.
It is, therefore, a still further object of this invention to provide an improved feedback control system for an engine having plural cylinders served by a common exhaust system and wherein the feedback control is varied on a cylinder-by-cylinder basis.